1. Field of the Invention
This invention concerns drill bit assemblies for drilling, coring or removing material from a geological subsurface formation.
2. Description of Related Art
Such drill bits have cutters which are either rigidly mounted on the bit body or on an extension of that body, e.g., blades or studs in the body, or may be mounted on roller cones which can rotate around axles rigidly fixed to the bit body. On the side of the drill bit usually distant from the cutters, such drill bits have a connector, usually threaded, which allows a rigid connection to be made between the bit and the bottom hole assembly and hence the drill string. In use the bit rotates and moves up and down. Eventually the bit is worn out or prematurely broken.
The replacement of a bit involves high cost in lost time as well as the cost of the new physical equipment. The problem of breakage of bits is thus very important in the drilling industry.
In relation to diamond faced bits for cutting or scraping such as diamond faced studs or faces, especially polydiamond crystal (PDC) wafer facing, the cutter comprising the diamond facing may become prematurely broken or dislodged. One reason for the breakage of PDC bits is that caused by vibration of the bit on the end of the very long drill pipe, the vibration resulting e.g. from interaction of the bit and the formation, or of the drill string and the well bore, and causing motion of the bit, which is not concentric nor at uniform speed, e.g. causing slip-stick, bit whirl and bit bounce.
Antiwhirl bits have been described and used in which the cutters are not uniformly distributed around the bit; in at least one place instead of a cutter there is a frictionless pad, the effect of which is that on contact of it and the rock, the bit slides over the rock surface instead of gearing with it. Although antiwhirl bits have in some cases enabled PDC bits to drill into harder formations, they have been less successful in highly interbedded formations, e.g. when drilling through rocks of variable or different hardness, which results in vibration of the bit. This problem is especially acute with exploratory wells where the nature of the rocks and the location of their interfaces is not accurately known. Because the cutters are in contact with different rocks, the resultant side force on the bit can no longer be maintained within the low friction pads so the low friction pads of the antiwhirl devices lose their effectiveness. There is thus vibration, an eccentric hole and breakage/dislocation of the cutter.
It is known to provide drill strings for driving drill bits having rotary drive transmitting sections which can be moved relative to one another from an axially aligned disposition in order to allow entering and drilling horizontal well sections, through a short radii curved hole, that would require excessive bending of a conventional stiff drill string. This may be achieved for example by providing hinged driving joints between the two sections or between the lower end of the drill string and the drill bit, or by providing wall sections which can be readily deformed to accommodate angular changes in the drilling direction. As the purpose of those devices is to cope with an important hole curvature, the drill bit itself is left rigid, in accordance with conventional bit designs.
EP-A-0,225,101 is concerned with reducing the problem of overheating of drill bits caused by excessive weight-on-bit during drilling or by sudden overload. This is achieved by a bit body having at least two relatively movable structures each carrying cutting elements, the two structures being relatively movable between two limiting positions to allow a change in configuration of the bit to be effected when required. In some embodiments resilient means may be provided to oppose relative movement of the structures in an axial and/or rotational direction. However there is no teaching or suggestion in this specification of providing any means for allowing tilting or relative lateral movement of the two relatively moveable structures of the drill bit assembly of EP-A-0,225,101.